Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones. General formula (CH2O)n and may contain N, S or P
• Roles of carbohydrates are:

  • Fuel or storage
  • Structural compounds
  • Recognition or regulation of extracellular proteins
  • Structural component in nucleic acids
• Monosaccharides, disacharides nad oligosaccharides are polymeric and generally soluble in water
• Homopolysaccharides or heteropolysaccharides are polymers of one or more carbohydrate units and are generally insoluble in water and are large molecules
• Glycoconjugates are carbohydrates joined to other molecular classes. Can be proteins or peptides like proteoglycans or glycoproteins or peptidoglycans or can be lipids like glycolipids
• Monosaccharides are:

  • Aldehydes or ketones with two or more hydroxyl groups.
  • Unbranched carbon chains with single bonds
  • If C1 is aldehyde it is an aldose
  • If C3 is ketone then is a ketose
• Epimers differ in configuration at only one chiral center
• D or L are classed by the bottom chiral center farthest from the carbonyl.

  • It is L if the OH of bottom chiral center is to the left
  • It is D if the OH of bottom chiral center is to the right
• Enantiomers are mirror reflection stereoisomers
• Hemiacetals are formed from alcohols and aldehydes when the C1 and last carbon curve to form cyclic structure
• Hemiketal is formed from alcohol and ketones
• Pyranoses are six membered rings
• Furanoses are 5 membered rings
• When a monosaccharide adopts a cyclic structure it forms new chiral center and converts carbonyl to hemiacetal for aldose or hemiketal for ketoses
• In solution 66% Beta arrangement is formed, 33% alpha arrangement is formed and 1 % linear formed for monosaccharides
• Monosaccharide is alpha is OH is on opposite side of rings as C6
• Monosaccharide is Beta if OH is on same side of ring as C6- ex both up
• Mutarotation is where alpha and beta forms may be interconverted by breaking and reforming hemiacetal
• If monosaccharide is in L conformation then alpha conformation OH would be Up and Beta conformation OH would be down
• The larger pyranose ring is more stable
• In Haworth projections an OH below the ring is on the right side of fischer projection
• Monosaccharides can be in chair conformations if pyranose and have two possible conformations that can be interconverted. OH groups are better to have in equatorial positions
• Aldonic acids have been oxidized and have carboxyl group at C1 have suffix of -onic acid
• Uronic acids have oxidation of primary alcohols so at C6 or the last carbon in the chain. Has suffix -uronic acid
• Alditols is a reduction of carnbonyl to alcohol so C1 changes from aldehyde to alcohol. Suffic -itol
• Removal of OH from C2 from monosaccharides gets deoxy sugars
• Replacement of OH at C2 to Nh2/NH3+ results in an -amine suffix

  The amino group can be acetylated to have N-acetyl-
• Glycosidic bonds form between two monosaccharides at one anomeric carbon to an OH group forming full acetals or ketals.
• Reducing sugars are ones with hemiacetals or hemiketals at anomeric carbon
• Nonreducing sugars have all anomeric carbons in glycosidic bonds
• N-glycosidic bonds connect nucleic acid bases to sugars
• O-glycosidic bonds connect anomeric carbons to alcohols
• Polysaccharides

  • also called glycans
  • O-glycosidic bond join sequential monosaccharides
  • Monosaccharide units, atoms involved in glycosidic bonds and configuration of anomeric carbons are what make them distinct
• Disaccharides are formed from two monosaccharides joined by glycosidic bonds.

  • It is anabolic process so has no energy output
  • It is an O-glycosidic bond between an OH of one saccharide ot the anomeric carbon of the other
  • Can be reducing or not
• Naming Disaccharides:

  • Begin at non-reducing end determine if confirguation is alpha or beta at anomeric carbon linkage in the middle
  • Name first monosaccharide including pyranose or furanse ending in -osyl
  • Name the the linkage numbers of both saccharides
  • Name the other monosaccharide including pyranose or furanose
  • The suffix -ose is added if reducing and -oside if nonreducing
• Polysaccharides are

  • Greater than 20 monosaccharides long
  • Vary by composition can be homo or hetero
  • Vary by structure can be branched have different glycosidic linkages and different lengths
• Homopolysaccharides

  • Have varying average molecular weights
  • Structures are determined by monosaccharides and glycosidic linkages- want to maximize H-bonding, minimize steric conflicts and can associate or exclude water
  • Are structural compounds like cellulose or chitin
  • Are for fuel storage like starch or glycogen
• What are the most abundant structural polysaccharides?

  Chitin and cellulose
• Cellulose is:

  • A linear chain of D-glucose residues joined by beta 1-->4 linkage
  • Can be 15,000 units long
  • Rigid and insoluble structure
  • There is a 180 rotation between sequential monosaccharide
  • H bonds form within and between chains
  • Chains align side by side to have extensive inter-molecular interactions
  • Form fibrous structure if excluding water
• Plants only have cellulose as animals do not have cellulase enzyme to catabolize beta 1-->4 linkages. Cellulase allows for cellulose to be used as energy source because it can break it down